1. Field of the Invention
This invention relates to spray drying a calcium carbonate-containing slurry into a granular matrix such that the ultrafine calcium carbonate particles maintain a high effective surface area and exhibit low agglomeration. Calcium carbonate-containing granules formed in the present invention may be used in the concurrently filed U.S. Application Ser. No. 509,866, filed Sept. 27, 1974 entitled BUILDER SYSTEM AND DETERGENT PRODUCT to Richard W. Benson, Steven D. Cherney, and Everett T. Collier, or in the detergent composition disclosed in Belgian Patent No. 798,856 to Jacobsen et al. both of which are herein incorporated by reference.
2. Description of the Prior Art Practices
This invention relates to the preparation of a spray-dried calcium carbonate-containing granule wherein the calcium carbonate particles within the granule retain a substantially discrete form and substantially the same surface area as the calcium carbonate particles had in the slurry before spray drying.
It is known that finely divided or microcrystalline calcium carbonate particles may be used as a crystallization seed in combination with a precipitating builder to accelerate the removal of soluble calcium ions from wash water. The term "soluble calcium ions" includes not only divalent calcium ions but also calcium salts which have associated but have not precipitated, for instance, singular calcium carbonate molecules. The associated forms of soluble calcium such as the monomolecular calcium carbonate exist in rapid equilibrium with calcium ions and carbonate ions such that depletion of both the free calcium ions and the other soluble associated calcium species is necessary.
Crystallization seeds such as those employed in the present invention are believed to function by providing growth sites for the precipitating free calcium ions and other soluble calcium species. The number of crystallization seeds present per unit weight and the surface area of each particle determines the total surface area present per unit weight. The number of growth sites upon which soluble calcium may precipitate appears to be directly proportional to the total surface area of the crystallization seed. The problem not previously solved is to economically incorporate the ultrafine or microcrystalline calcium carbonate particles into a granular detergent composition such that the particles are maintained in their discrete or nonagglomerated high effective surface area form thereby requiring a minimal amount of calcium carbonate. The term "effective surface area" indicates a measurement of the surface area available for crystal growth which is area available to precipitate soluble calcium onto a given weight of calcium carbonate crystallization seeds. The term "nominal surface area" indicates the total surface area of a given weight of calcium carbonate crystallization seed without regard to the effectiveness in soluble calcium depletion. To provide favorable kinetics for soluble calcium removal from a wash water solution, it is desirable that the calcium carbonate crystallization seeds have a high effective surface area per unit weight and a large number of particles.
Several methods are known in the art for the preparation of calcium carbonate particles which in slurry form exhibit high surface areas and particle diameters within the range desirable for incorporation into laundry detergent compositions. The known methods for maintaining large numbers of discrete calcium carbonate particles of a high nominal surface area in a slurry include the addition to the aqueous slurry of a material such as sodium tripolyphosphate, sodium pyrophosphate, soluble silicates, and various anionic detergents. The problem with using the aforementioned materials to maintain a high nominal surface area and a large number of calcium carbonate particles is that the effective surface area is substantially reduced by the adsorption of the aforementioned salts and detergents onto the crystal surfaces. The apparent effect of, for example, polyphosphate ions is to bind to the lattice of the calcium carbonate crystallization seed to form a stable layer onto which the precipitation of the soluble calcium will not occur. The rendering of a crystallization seed ineffective with respect to the growth or nucleation sites such as by adsorption is known as poisoning.
The calcium carbonate crystallization seeds within an aqueus slurry when dried agglomerate resulting in a substantial loss of the number of discrete particles present as well as a reduction in the effective surface area of the calcium carbonate particles. However, the drying of a slurry of calcium carbonate particles while substantially reducing the effective surface area may in many instances reduce the nominal surface area only slightly. The phosphate salts, silicates, and anionic surfactants, which were previously mentioned as having the ability to maintain a large number of particles in a slurry do not in any case at the levels disclosed prevent the agglomeration of the calcium carbonate crystallization seed particles upon drying.
It is intended throughout the specification and claims that the term "agglomeration" or "agglomerate" is to embrace aggregate or composite which are more fully defined in U. S. Pat. No. 2,964,382 entitled PRODUCTION OF PRECIPITATED CALCIUM CARBONATE and issued to G. E. Hall on Dec. 13, 1960.
For the purposes of this invention, the effective surface area and the nominal surface area are equivalent for a given slurry of nonpoisoned, non-agglomerated calcium carbonate particles. It is intended throughout the specfication and claims that the terms free calcium ions, soluble calcium, and soluble calcium species, may be used interchangeably.
It has now been discovered that an aqueous slurry of discrete submicron high surface area calcium carbonate particles can be spray dried to give a calcium carbonate-containing granule wherein the calcium carbonate particle retains its high effective surface area and has a lessened tendency towards agglomeration or aggregation when an alkali metal carbonate, bicarbonate, or sesquicarbonate is included in the slurry.
As a result of practicing this invention, it is possible to spray dry a calcium carbonate-containing slurry to give a calcium carbonate-containing granule which will rapidly dissolve upon addition to a wash water solution and present a calcium carbonate crystallization seed of high effective surface area onto which soluble calcium ions in the wash solution may precipitate.
As was mentioned above, it is desirable that the calcium carbonate particles present in a wash solution exhibit a high effective surface area substantially equivalent to the nominal surface area of the calcium carbonate particles present in the slurry prior to spray drying. It will also be observed that by practicing the present invention that the tendency of the calcium carbonate particles to agglomerate during spray drying is substantially lessened as evidenced again by the calcium depletion rates of the non-dried slurry of calcium carbonate particles and the similar measurements made when the spray-dried calcium carbonate-containing granule is dissolved in a wash solution. Rapid soluble calcium depletion is necessary to prevent the soluble calcium from interfering with the detergent or depositing upon the fabric as calcium carbonate.
It has also been observed that previous attempts to use finely-divided or microcrystalline calcium carbonate particles in detergent compositions is objectionable because of the dust problems which can occur in handling calcium carbonate as a substantially dry material.
Another deficiency resulting from the incorporation of calcium carbonate crystallization seeds into detergent products is that the calcium carbonate particles during handling and shipping tend to segregate from the material with which they are admixed. It is further noted that when calcium carbonate crystallization seed particles are admixed directly into the granular detergent product that these particles when stored under conditions of high temperature and humidity have a tendency to cluster together to give a caked product.
It is an object of the present invention to prepare a calcium carbonate-containing granule from a calcium carbonate-containing aqueous slurry such that the discrete nature and the high effective surface area of the calcium carbonate particles in the slurry are substantially retained by the calcium carbonate particles within the spray-dried granule.
It is a further object of the present invention to prepare a calcium carbonate crystallization seed-containing granule which will rapidly dissolve when added to a wash solution such that the high surface area discrete calcium carbonate crystallization seed particles are rapidly available to induce the removal of soluble calcium ions from the wash solution.
It is yet a further object of the present invention to provide a method for the incorporation into a detergent product of submicron calcium carbonate crystallization seeds in the form of a much larger calcium carbonate-containing granule which substantially reduces the dust problems incurred in admixing dry submicron calcium carbonate particles.
Another object of the present invention is to provide a method for the incorporation into a detergent product of submicron calcium carbonate crystallization seeds such that the crystallization seeds do not cake together under conditions of high humidity and temperature.
It is yet another object of the present invention to provide a suitable carrier granule for certain detergent components such as soil suspension agents.
The benefits of the present invention are further described in the concurrently-filed, copending U.S. Application Ser. No. 509,864, filed Sept. 27, 1974, by Cherney, entitled SPRAY-DRIED CALCIUM CARBONATE-CONTAINING GRANULES herein incorporated by reference.